Field-Configurable Rectification in CO-FePhthalocyanine Single-Molecule Junctions

Abstract

Single-molecule junctions represent the ultimate limit of electronic miniaturization, where quantum mechanical effects and molecular orbital symmetry directly determine device behavior. This thesis investigates the transport properties of CO–FePhthalocyanine molecular junctions from a theoretical and computational perspective, establishing a quantitative framework for understanding and predicting rectification in transition-metal phthalocyanine systems. Using a Landauer transport model parameterized by density functional theory values and scanning tunneling spectroscopy measurements from the literature, a standard approach in molecular electronics that combines physically grounded parameter estimates with an interpretable trans port framework, we compute the current–voltage characteristics of CO–FePc and bare FePc junctions across a systematic variation of orbital energy, coupling asymmetry, tip–molecule distance, temperature, Fe charge state, and CO orientation. The central finding is that CO enhances the rectification ratio R = |I(−V )|/|I(+V )| by a factor of 3.10 relative to bare FePc, producing R = 9.18 at reference conditions of |V | = 1.0V and T = 50K, through two independent mechanisms. First, the σ-donation interaction shifts the Fedz2 orbital from −0.40eV to −0.15eV relative to the electrode Fermi level, enhancing the low-bias conductance G by a factor of 6.71, compared to an experimentally observed enhancement factor of approximately 3. Second, the permanent CO dipole moment couples to the junction electric field via a linear Stark shift, generating a bias-dependent asymmetry parameter α = αgeom + αdip, where αgeom reflects the asymmetric tip–substrate coupling and αdip reflects the CO dipole Stark shift that drives the transmission resonance preferentially through the Fermi window under negative tip bias. Reversing the CO orientation from carbon end-down to oxygen-end-down reverses the sign of αdip, switching the junction from a robust forward rectifier (R = 9.18) to a weak reverse rectifier (R = 0.945) while leaving G unchanged, a consequence of the orientation-independence of σ-donation. The two orientations are spectroscopically distinguishable by the sign of their differential conductance dI/dV peak voltage: −0.155V for C-down and +0.210V for O-down. The rectification ratio degrades by only 19% from 4K to 300K, reaching R = 7.43 at room temperature, a consequence of the Γ-broadened transport regime in which thermal smearing is a perturbation on the dominant coupling broadening Γ = ΓL+ΓR, where ΓL and ΓR are the tip and substrate coupling strengths, respectively. A Marcus theory model for Fe(II)/Fe(III) charge-state switching predicts bistable hysteretic I–V characteristics with a threshold bias sweep rate of u∗ ≈ 0.08V/s, establishing that the junction functions simultaneously as a molecular diode and a memory element. A rectification phase diagram computed across the full (α, ΓL/ΓR) parameter space demonstrates that the sign of α is the primary determinant of rectification direction, and that CO orientation control moves the operating point across the R = 1 flip boundary without any chemical modification of the molecule or electrode. These results establish CO–FePhthalocyanine as a field-configurable molecular device whose rectification magnitude and direction are continuously tunable through junction geometry, axial ligand chemistry, and atomic-scale CO manipulation.